High voltage dry-type reactor for a voltage source converter

ABSTRACT

A high voltage dry-type reactor is series-connected via a first terminal to an AC supply voltage and via a second terminal to the AC phase terminal of a high voltage converter and includes a cylindrical coil of insulated wire. In order to protect the reactor from a damaging DC field, the reactor further includes a metallic or resistive electrostatic shield which is connected to a same DC potential as the converter.

The invention relates to a high voltage dry-type reactor which isseries-connected via a first terminal to an AC supply voltage and via asecond terminal to the AC phase terminal of a high voltage AC/DC orDC/AC converter and which comprises a cylindrical coil of insulatedwire. The converter is preferably a voltage source converter used in ahigh voltage direct current (HVDC) power transmission system.

In today's power transmission and distribution systems, reactors areused to introduce an inductive reactance into the correspondingelectrical circuit. A reactor can also be called an inductor. Its maincomponent is a coil of insulated wire which can either be wrapped arounda core of magnetic material, i.e. an iron core, or can be constructed inthe form of a hollow body, i.e. a hollow cylinder or a hollow cuboid,with no magnetic material inside. The latter group of reactors is knownas air-core reactors.

Reactors are used in power systems for example as filter reactors tofilter out undesired harmonics in a current transmitted to a powernetwork, as shunt reactors to compensate for capacitive reactive power,as neutral-grounding reactors to limit the line-to-ground current of adirectly earthed network or as current-limiting reactors to limitshort-circuit currents.

The winding of a reactor used under high-voltage and high-currentconditions of a power system produces considerable heat. Therefore,appropriate cooling is necessary to reduce the temperature in thereactor coil in order to minimize the losses and to avoid thermal ageingof the insulating material. The cooling of an air-core reactor can beprovided by insulating the reactor coil in a cooling fluid or by lettingair flow alongside the coil windings. Air-cooled reactors are also knownas dry-type reactors.

In high voltage direct current (HVDC) power transmission systems, poweris transmitted between two AC power networks which are connected via aDC link. Accordingly, an AC/DC and a DC/AC converter are installed atone side of the DC link, respectively. The converters can be either ofline commutated converter type or of voltage source converter type. Incase of a line commutated converter, a reactor is used to remove currentripples on the DC side of the converter. This reactor is called asmoothing reactor. When voltage source converters are used in the HVDCsystem, additionally a reactor called converter reactor or phase reactoris used on the AC side of the converter to mainly block harmoniccurrents arising from the switching of the converter. Apart fromblocking harmonic currents, the converter reactor serves the additionalpurposes of providing active and reactive power control and limitingshort-circuit currents. Both reactor types and their arrangement in anHVDC system are for example known from the brochure “It's time toconnect”, issued by ABB Power Technologies AB, Grid Systems-HVDC, SE-77180 Ludvika, Sweden, www.abb.com/hvdc.

The present invention deals with a converter reactor, i.e. a reactorconnected in series to the AC side of a high voltage AC/DC or DC/ACconverter, preferably a voltage source converter. Such converterreactors are usually dry-type reactors, i.e. no insulating oil is used.

A commonly known AC/DC or DC/AC part of a HVDC system with voltagesource converter is shown in a single-line diagram in FIG. 1. Usually,the AC part of a HVDC system contains three phases. A voltage sourceconverter (VSC) 1 comprises converter valves 2 connected in a knownbridge configuration, where the converter valves 2 each comprise an IGBT3 (Insulated Gate Bipolar Transistor) in anti-parallel connection with afree-wheeling diode 4. The VSC 1 is connected on its AC side to aconverter reactor 5, followed by a harmonic filter 6 and a transformer7. The transformer 7 is coupled to an AC power network 8. Two identical,series-connected capacitor units 9 are connected between a first pole 12and a second pole 13 of the DC side of the VSC 1, and the DC link of theHVDC system is in this example made up of two DC cables 10. The DCcables are insulated and their shields are grounded. Instead of DCcables, overhead lines may be used as well. The connection point 11between the two capacitor units 9, also called the midpoint ormidpotential of the DC side of VSC 1, is grounded, so that a symmetricalDC voltage occurs between the two poles 12 and 13. Accordingly, the DCcable 10 connected to the first pole 12 has a positive voltage potential+U_(DC,1) and the DC cable 10 connected to the second pole 13 has anegative voltage potential −U_(DC,1) with the same absolute value as thepositive voltage potential.

New developments in HVDC technology suggest an asymmetric system, whereinstead of the midpoint between the capacitor units 9 one of the poles12 or 13 is grounded. In FIG. 2, such an asymmetric system is shownwhere the first pole 12 is connected to ground and the second pole 13 isconnected to a DC cable 10. The voltage potential on the DC cable 10 isnegative (−U_(DC,2)) and has usually a different value than the negativevoltage potential (−U_(DC,1)) in the symmetrical configuration ofFIG. 1. Since a DC cable constitutes a considerable cost factor in aHVDC system, a reduction from two to one cable results in a major costreduction. A similar asymmetric configuration can be set up using onlyone overhead line instead of two, the remaining pole being connected toearth. This solution would not only mean that less material is neededfor the overhead lines, but it would also result in the reduction oftransmission losses since the earth has a much smaller resistance thanan overhead line.

It is an object of the present invention to provide a converter reactorwhich is suitable to be used in the asymmetric configuration of an HVDCsystem.

The object is achieved by the characterizing features of claim 1.

The invention is based on the recognition of a fundamental problemarising in the asymmetric configuration. The problem is caused by thefact that an asymmetric configuration of the HVDC system results in a DCoffset on the AC side of the VSC 1, which is opposed to the symmetriccase where no DC offset occurs. The DC offset results in a DC electricfield between the converter reactor 5 and ground which leads to theaccumulation of charges on the insulating outer and inner surfaces ofthe reactor 5. This situation is depicted in FIG. 3 a, where a converterreactor 5 is shown schematically, comprising a cylindrical coil ofinsulated wire 14 which is surrounded by an insulating cylinder 15. Theinsulating cylinder 15 is placed on two insulators which stand on aground 17. The winding of the coil 14 is electrically connected on oneside via a first terminal A to a connection point on the secondary sideof the transformer 7 (see FIG. 2) and on the other side via a secondterminal B to the AC phase terminal 35 of converter 1. Accordingly, theterminal A sees the DC offset potential plus the AC voltage of thesecondary transformer side and the terminal B sees the DC offsetpotential plus the switching voltage of the converter 1. Since the DCpotential is of negative value in the example of FIG. 2, the resultingcharges 18 on the surface of the insulating cylinder 15 are positive.The charges 18 accumulate not only on the outer surface of theinsulating cylinder 15 as in FIG. 3 a, but also on its inner surfacefrom where they may affect the winding of coil 14. FIG. 3 b shows across section of some turns of the wire 19 of coil 14. The wire 19 issurrounded by a thin layer 20 of insulating material. This insulatinglayer 20 is usually thick enough to withstand the normal AC electricfields of a symmetric HVDC system, but in the asymmetric case, theincreased field strength could lead to puncturing, i.e. flashes throughthe insulating layer 20, which would damage the insulating material. Thecharges 18 could propagate between the windings and finally lead to thedestruction of the reactor or even a fire.

In order to prevent the damaging of the converter reactor caused by theDC field, the invention suggests to install a metallic or resistiveelectrostatic shield at the reactor, where the shield is connected to asame DC potential as the converter. The connection can be made to eitherthe DC side or to the AC side of the converter. On the AC side,terminals A or B are chosen since they see the converter's DC potentialas explained above. The shield eliminates the DC field around theconverter reactor and thereby prevents the appearance of dangerouscharges on the surface of the reactor winding. Puncturing anddestruction of the converter reactor can effectively be avoided,accordingly.

The invention is now described by way of example with reference to theaccompanying drawings in which:

FIG. 1 shows a known symmetric HVDC system;

FIG. 2 shows a known asymmetric HVDC system;

FIG. 3 a shows the charging of a converter reactor in an asymmetric HVDCsystem;

FIG. 3 b shows the charging of the turns of the converter coil of FIG. 3a;

FIG. 4 shows a converter reactor with a metallic cage connected to a DCpotential on the DC side of the converter;

FIG. 5 shows a converter reactor with a metallic cage connected to a DCpotential on the AC side of the converter via a first terminal;

FIG. 6 shows a converter reactor with a metallic cage connected to a DCpotential on the AC side of the converter via a second terminal;

FIG. 7 shows a converter reactor with a metallic cage, which ishigh-frequency connected to ground;

FIG. 8 shows a converter reactor with corona rings.

A first embodiment of the invention is shown in FIG. 4. There, theconverter reactor 5 of FIG. 3 a is enclosed in a metallic cage 21, whichis lifted via insulators 22 above ground 17. The metallic cage 21 is ahollow body which can be cylindrical or of any other three-dimensionalshape, and which has a bottom and a roof. The cage 21 can for example bemade of sheet or meshed metal or of wires with different profiles.Bushings 23 and 24 are led through the wall of the cage 21 in order toconnect the ends of coil 14 from the outside to the connection pointscorresponding to terminals A or B, respectively. The cage 21 iselectrically connected to a DC potential on the DC side of theconverter, which is in this special embodiment the midpotential atmidpoint C. The connection to the DC potential is beneficial from thepoint of view of possible radio interference. Resistors 25 are connectedin parallel to the capacitors 9 on the DC side of converter 1, and themidpoint of their series connection is connected to midpoint C in orderto stabilize the DC voltage distribution. In a further embodiment of theinvention, the metallic cage 21 is identical with a magnetic shield ofthe converter reactor 5, i.e. the cage 21 fulfils two functions at thesame time: it eliminates the DC field on the reactor coil 14 and itmitigates or eliminates magnetic fields outside the reactor.

In FIG. 5, the reactor 5 of FIG. 3 a is again enclosed in the metalliccage 21 of FIG. 4. Instead of connecting the cage 21 to a DC potentialon the DC side of the converter, the cage 21 is connected directly toterminal A of the reactor and thereby to the connection point on thesecondary side of transformer 7 in FIG. 2. Bushing 23 as well as theconnection to the point C (FIG. 4) is thereby omitted. FIG. 6distinguishes from the embodiment in FIG. 5 only in that the cage 21 isconnected to terminal B instead of terminal A and is thereby connectedto the AC phase terminal of converter 1 (FIG. 4). In this embodiment,bushing 24 as well as the connection to the point C (FIG. 4) is omitted.

FIG. 7 shows another embodiment of the invention. The arrangement isalmost identical to the one of FIG. 5. The only difference is that thecage 21 is connected via a resistor 36 to the potential of terminal A ofthe reactor 5 and via a capacitor 26 to ground 17. The time constant ofthe resistive-capacitive connection is preferably chosen in the range ofseconds or larger, and it establishes a strong high-frequency couplingof the cage to ground in order to mitigate high frequency voltagedisturbances arising from the switching of the converter valves 2.Alternatively, the high-frequency coupling of FIG. 7 could also beapplied to the embodiments of FIG. 4 or 6.

A still further embodiment according to FIG. 8 is not based on ametallic cage, but instead two first corona rings 27 and 33 are eachplaced around one of the two ends of the cylinder of coil 14.Additionally, two second corona rings 28 and 34 are each placed inparallel to one of the two end surfaces 29 of the cylinder of coil 14. .Corona rings 33 and 34 are electrically connected to the first terminalA of the reactor 5 and the other corona rings 27 and 28 on the oppositeend of coil 14 are electrically connected to the second terminal B ofthe reactor 5. The four corona rings 27, 28, 33 and 34 are all placed sothat the longitudinal axis 30 of the cylinder of coil 14 and the centralaxis of the rings are in line with each other. Each of the first coronarings 27 surrounds the shell 31 of the cylinder of coil 14 at a distanced_(e1) or d_(e2) from the respective end surface 29 which is shorterthan the respective distance d_(m1) or d_(m2) from the lateral middleaxis 32 of the coil cylinder. By placing the four corona rings 27, 28,33 and 34 on top and on the outer sides of the coil 14 and thereby thereactor 5, it is prevented that any charges flow into the interior ofthe reactor 5. In a special embodiment, the corona rings are arranged toreduce the flow of induced currents inside the rings in order to avoidexcessive magnetic heating. This is achieved by using a highly resistivematerial and/or by choosing a cross-section for the rings, whichencloses as little magnetic field as possible.

1. A high voltage dry-type reactor which is series-connected via a firstterminal to an AC supply voltage and via a second terminal to an ACphase terminal of a high voltage converter being part of an asymmetricconfiguration of a high voltage direct current system, the reactorcomprising: a cylindrical coil of insulated wire, and a metallic orresistive electrostatic shield which is connected to a same DC potentialas a DC offset potential on the AC side of the converter.
 2. The reactoraccording to claim 1, wherein the electrostatic shield comprises twofirst corona rings, each placed around one of the two ends of thecylinder of the coil so that the longitudinal axis of the cylinder andthe central axis of the rings are in line with each other and so thateach of the rings surrounds the shell of the cylinder at a distance fromthe respective end surface of the cylinder which is shorter than thedistance from the lateral middle axis of the cylinder, wherein one ofthe two first corona rings is electrically connected to the firstterminal of the reactor, and wherein the other of the two first coronarings is electrically connected to the second terminal of the reactor.3. The reactor according to claim 2, wherein the electrostatic shieldfurther comprises two second corona rings, each placed in parallel toone of the two end surfaces of the cylinder of the coil so that thelongitudinal axis of the cylinder and the central axis of the rings arein line with each other, wherein one of the two second corona rings iselectrically connected to the first terminal of the reactor, and whereinthe other of the two second corona rings is electrically connected tothe second terminal of the reactor.
 4. The reactor according to claim 1,wherein the corona rings are arranged to reduce the flow of inducedcurrents inside the rings by choosing a cross-section for the rings,which encloses as little magnetic field as possible, and/or by using ahighly resistive material.
 5. The reactor according to claim 1, whereinthe electrostatic shield comprises a metallic cage which surrounds thecoil and which is connected to a DC potential on the DC side of theconverter.
 6. The reactor according to claim 5, wherein the DC potentialis the midpotential on the DC side of the converter.
 7. The reactoraccording to claim 1, wherein the electrostatic shield comprises ametallic cage which surrounds the coil and which is connected to thefirst terminal of the reactor.
 8. The reactor according to claim 1,wherein the electrostatic shield comprises a metallic cage whichsurrounds the coil and which is connected to the second terminal of thereactor.
 9. The reactor according to claim 5, wherein the cage is via ahigh voltage resistor coupled to the first or second terminal of thereactor and is high-frequency coupled to ground.
 10. The reactoraccording to claim 9, wherein the high frequency coupling comprises theseries connection of the high voltage resistor and a high voltagecapacitor, the resistor being connected between the cage and the reactorand the capacitor being connected between the cage and ground.
 11. Thereactor according to claim 1, wherein the converter is a voltage sourceconverter for a high voltage direct current power transmission system inasymmetric configuration.